Electrostatic printing



Ocf- 4, 1966 R. H. FISHER ELECTROSTATIC PRINTING Filed April 13. 1959 MSSS 33. .$58k

INVENTOR. RUBERT I-I. FISHER BY Hams/r United States Patent O 3,276,896 ELECTROSTATIC PRINTING Robert H. Fisher, Trenton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Apr. 13, 1959, Ser. No. 805,761 12 Claims. (Cl. 117-37) This invention relates generally to electrostatic printing. More particularly, it relates to imp-roved methods and apparatus for developing electrostatic images.

In the art of electrostatic printing, electrostatic images are produced `on the surface of an insulating material. Such images comprise a pattern of electrostatic charges on the surface. Visible images are commonly produced therefrom by the cascading across the surface a dry mixture of finely divided developer particles and substantially larger carrier particles. When the developer particles are triboelectrically charged in the opposite polarity to the electrostatic charges, they deposit in charged areas to produce a visible image in substantial configuration with the pattern of charges. The foregoing method of developing electrostatic images is described in Elec trofax Direct Electrophotographic Printing .on Paper, by C. l. Young and H. G. Greig, RCA Review, December 1954, vol. XV, No. 4. Also described inV that publication are other methods of development such as: powder cloud, liquid -mist and magnetic brush types.

The recording element may `comprise almost any insulating surface but, preferably, the recording surface is also photoconductive to enable the recording of light images. Recording elements comprising photoconductive selenium coated plates are described in U.S. Patent 2,297,- 691, issued October 6, 1942 to C. F. Carlson. Recording elements comprising photoconductive coatings on -paper are described in the Young and Greig publication, op. cit.

Recently, a sor-called liquid process for developing electrostatic images has been proposed in which the solid developer particles are suspended in an insulating carrier liquid. Liquid development methods provide many distinct advantages over the use of dry developer mixtures and other methods of developing electrostatic images, for some applications. Basically the liquid developer consists of finely divided developer particles dispersed in an insulating liquid. This developer can be flowed over a surface bearing an electrostatic image or, the surface can be immersed in a tray of liquid developer. It can also be sprayed `or rolled on to the surface. When appropriate developer particles are dispersed in a properly selected liquid, they acquire an electrophoretic or triboelectric charge enabling them to be attracted to an electrostatic charge pattern of appropriate polarity. Deposition of the developer particles on the charge image is an example of the phenomenon known as electrophoresis or cataphoresis. A liquid developer process for charge images is described in greater detail by K. A. Metcalf and R. l. Wright in a paper entitled, Xerography, published in the Journal of the Oil and Colour Chemists Association, November 1956, volume 39, No. 11, Lond-on, England, and in another paper entitled, Liquid Developers for Xeiography, published in the Journal of Scientific Instruments, February 1955, volume 32.

3,276,896 Patented Oct. 4, 1966 Although the above-mentioned liquid development processes lare suitable for many purposes, it has been found that they have at least one serious deficiency. When producing a visible image by deposition of developer particles from a liquid onto a charged surface, particles adhere in the background areas as Well as in the image areas on the surface. Background areas are those areas on the surface bearing little o-r no electrostatic charge and hence, vare those areas in which deposition of developer particles is unwanted. Such spurious deposit in non-image areas results in a spottedor mottled background on the finished print which, in many applications, is entirely unsatisfactory. This effect is even more pronounced when attempting to Ireproduce fine detail. The more finely divided the developer particles are, the greater the spurious deposition in background areas.

It is a general object of this invention to provide improved methods for liquid development of electrostatic images.

Another `object of this invention 4is to provide improved methods of developing electrostatic images with liquid dispersions which minimize adherence of developer material in non-image areas of the finished print.

A still further object of this invention is to provide improved apparatus for accomplishing the foregoing objects.

These and other objects are accomplished in accordance with the methods of this invention which, in general, provide for removal of developer particles spuriously deposited from a carrier liquid in background areas of an electrostatic image. The method comprises, first, applying to an electrostatic image on an insulating surface a developer mixture ycomprising electroscopic developer particles dispersed in an insulating carrier liquid to produce a visible image of `developer particles `on the insulating surface and then applying relatively clear carrier liquid to the surface to remove developer particles from non-image areas thereon. When desired, thermoplastic developerwparticles may be employed in the developer mixture making it possible to permanently fuse the visible image to the insulating surface by applying heat thereto.

Apparatus which may be employed in carrying out the methods of this invention is also provided. This apparatus comprises means for applying to the insulating surface a developer mixture of elect-roscopic developer particles dispersed in an insulating carrier liquid, Washing means for subsequently applying to the surface carrier liquid relatively devoid of developer particles and means for providing relative motion of said surface with respect to both the means for applying developer mixture and the washing means such that the application of carrier liquid relatively devoid of developer particles occurs subsequent to the application of developer mixture.

Other objects and advantages are included in the following detailed description to be read with reference to the accompanying drawings wherein:

FIGURE 1 is a flow chart of a complete electrophotographic 4process embodying the methods of this invention.

FIGURE 2 is a partially schematic cross-sectional view of a preferred developer apparatus embodying the methods of this invention.

The How chart of FIGURE 1 illustrates an electrophotographic process in which the methods of `this invention are particularly applicable. The first step of the electrophotographic process comprises producing a blanket electrostatic charge on a photoconductive surface. One suitable type of recording element comprises a paper backing on which there is coated a dispersion of particulate photoconductivity zinc oxide in a resinous filmforming vehicle. This coated paper is conveniently charged by passing over the photoconductive surface one or more times a corona discharge device operating at a potential of about 6000 volts. The next step of the process comprises exposing the charged photoconductive surface to a light image. This may be done either by typical contact exposure or projection exposure techniques commonly employed in the photographic arts. Photoconductive recording elements, charging and exposing are more fully described in Electrofax-Direct Electrophotographic Printing on Paper, by C. J. Young and H. G. Greig, RCA Review, volume 15, No. 4.

In accordance with this invention a `two-step liquid develompent technique is employed to render the electrostatic image, produced as a result of the exposure step, into a visible image. First, a developer mixture comprising finely-divided `developer particles dispersed in an insulating liquid is applied to the photoconductive surface. This may be accomplished by immersing the recording element in the mixture or -by rolling the mixture onto the surface. Suitable mixtures for development are described in the Metcalf and Wright publication op. cit.

A preferred `developer mix comprises thermosetting resinous particles dispersed in a carrier fluid such as silicone liquid. The developer particles are prepared by melting 200 parts by weight of a resin such as, for example, Piccolastic 4358A, an elastic thermoplastic resin composed of polymers of styrene, substituted styrene and its homologs, marketed by the Pennsylvania Industrial Chemical Co., Clairton, Pa. Mixed into the melted resin are 12 parts by weight of carbon black, 12 parts spirit nigrosine and 8 parts Iosol Black. When a uniform mixture is obtained, it is cooled and ground to a tine powder. Twenty parts by weight of developer powder is dispersed in 80 parts by weight of dimethyl polysiloxane uid having a viscosity of about 2 centistokes.

The second portion of this development method comprises applying to the photoconductive surface a relatively clear carrier liquid after the application thereto of the developer mixture. This again may be accomplished by spraying or rolling or by immersing the recording element in the liquid. When the clear carrier liquid is moved across the surface it picks up ydeveloper particles which have been accidentaly `deposited in background or nonimage areas on the photoconductive surface. In most applications, such background deposit is objectionable and it has been found impossible `to completely avoid such deposition during the first portion of this development procedure. Removal of spurious deposit may be accomplished with a clear carrier liquid or with one which contains a relatively minor amount of l'developer particles. Thus, one may have a tray of carrier liquid in which the `recording element is immersed. Developer particles washed from rthe photoconductive surface will flow into the tray and after a considerable length of time will be present in such quantity as to reduce the effectiveness of the washing step. However, it has been found that the carrier liquid can be used many times before the concentration of developer particles therein becomes too great.

Once development and washing have been completed it may be desirable to fuse the image onto the photoconductive surface to avoid smearing. This may be easily accomplished by heating the recording element to a temperature above the melting point of the developer particles to provide a permanent fused image on the photoconductive surface.

In accordance with the foregoing methods of this invention a clear detailed image may be produced wherein spurious deposit of developer particles in background areas is minimized.

In FIGURE 2 there is illustrated a preferred embodiment of an apparatus for carrying out the methods of this invention. Electrophotographic recording paper 10 passes under a transport roller 12. The transport roller preferably has a resilient surface such as rubber. Adjacent to the tansport roller is a developer roller 14 positioned with respect to the transport roller 12 such that the paper 10 passes close to the developer roller 14. Also positioned close to the transport roller 12 is a washing roller 16. These two rollers 12 and 16 are in virtual contact and rotate at the same speed so as to provide the motive power for transporting the paper 10. The developer roller 14 is mounted in developer tray 18 so as to rotate therein with its bottom periphery well down into the tray. This developer roller is preferably of metal and has a fiuted surface designed to agitate a developer mixture 20 contained in the tray 13. During rotation the tluted surface of the developer roller 14 carries liquid up from the tray and applies it to the surface of the paper 10. This roller is preferably spaced with its surface about 1/16 of an inch from that of the transport roller 12. Because of this spacing, liquid carried out by the uted surface of the developer roller 14 will form a puddle of developer mixture, as shown at 22, in contact with the paper 10. Although it is not critical, it has been found that the spacing of j/1(; provides for maximum deposi- .tion of developer particles on the paper 10. As shown in the drawing, the forming of the puddle 22 maintains developer mix in contact with the paper over a finite distance thereby providing more time for the electrostatic image on the paper to be effective in attracting developer particles thereto.

Once development of an image on the paper 10 has been accomplished, the paper is caused to pass between the transport roller 12 and the washing roller 16 having a relatively smooth surface such as, for example, polished stainless steel. Since these two rollers are in virtual contact, they act to squeegee excess carrier liquid from the surface of the paper 10. In addition to the foregoing, the washing roller 16 is mounted in the developer tray 18 so that it rotates therein at a considerably lesser depth than does the developer roller 14. yBecause of the smooth surface of the washing roller 16, the developer mix in which it rotates will undergo little or no agitation, thus permitting developer particles' squeegeed from the paper 10 to settle toward the floor 24 of the tray 18. Due to this settling action and also because the squeegeeing action of washing roller 16 is causing substantially clear liquid to be introduced into the tray in its vicinity, washing roller 16 rotates in substantially clear carrier liquid. During its rotation relatively clear carrier liquid is picked up by the washing roller 16 which, along with the relatively clear liquid squeegeed `from the paper 10, forms a puddle of substantially clear carrier liquid, as shown at 23. This puddle of clear carrier liquid removes' substantially all developer particles from background areas of the developed image on the paper 10.

Also, since the washing roller 16 rotates in relatively clear carrier liquid, developer particles adhering to the surface of the roller after the paper 10 has been squeegeed are washed from the roller when it rotates through the liquid thereby substantially eliminating the risk of offset printing of ghost images by the washing roller 16, itself.

The shape of the developer -tray 18 is not essential. A plane rectangular tray has been found to function satisfactorily. However, development speeds may be increased where one employs a tray having substantially the shape depicted in FIGURE 2. This tray has a slanting floor 24 which is shallower at the end adjacent the washing roller 16 than at the end adjacent the developer roller 14. This configuration facilitates concentration of developer particles in the deeper portion of the tray adjacent the developer roller 14. The particular value of this configuration is eviden-t when development is first initiated, since at this time, developer particles frequently have settled in a uniform manner toward the bottom of the tray. The slope of they bottom of the `tray insures that upon initiating development, developer particles will be concentrated in the vicinity of the developer roller 14 and that the washing roller 16 will be immersed in relatively clear carrier liquid.

There have bee-n disclosed new methods of developing electrostatic images and of apparatus for accomplishing such development. The methods and apparatus of this invention makes it possible to produce visible images with more clarity and definition and with a minimum of background deposition than have been hitherto obtainable employing liquid development techniques.

What is claimed is:

1. I'n electrostatic printing a method of developing an electrostatic image on an insulating surface comprising the steps of applying to said surface a mixture comprising electroscopic developer particles dispersed in an insulating carrier liquid whereby said developer particles are attracted to said electrostatic image and then removing developer particles adhering to said surface in non-image areas thereof by washing said surface with insulating carrier liquid which is relatively devoid of developer particles.

2. The method of claim 1 including the additional step of squeegeeing said insulating surface to enhance removal of developer particles from non-image areas thereon and to remove carrier liquid therefrom.

3. A method according to claim 1 in which said mixture is applied to said surface from a metallic applicator.

4. A method according to claim 3 including the additional step of squeegeeing said insulating surface to remove carrier liquid therefrom.

5. In electrostatic printing a method of developing an electrostatic image on an insulating surface comprising the steps of applying to said surface a developer mixture comprising electroscopic thermoplastic developer particles dispersed in a non-inflammable insulating carrier liquid to produce a visible image of said developer particles on said surface, removing developer particles adhering in non-image areas on said surface by washing said surface with insulating carrier liquid which is relatively devoid of developer particles, and heating said surface to fuse said visible image thereon.

6. In electrostatic printing the method of developing an electrostatic image on an insulating surface comprising the steps of applying to said surface a developer mixture of developer particles at least the surfaces of which comprises electroscopic thermoplastic resinous material dispersed in an electrically-insulating silicone liquid to produce on said insulating surface a visible image of said developer particles, removing developer particles from non-image areas on said insulating surface, squeegeeing said insulating surface to remove silicone liquid therefrom and enhance the removal of developer particles from nonimage areas thereon by washing said surface with silicone liquid which is relatively devoid of developer particles, and heating said insulating surface to fuse said visible image thereto.

7. In electrostatic printing a method of developing an electrostatic image on an insulating surface comprising the steps of applying to said surface a developer mixture comprising electroscopic thermoplastic developer particles dispersed in an insulating carrier liquid to produce a visible image of said developer particles on said surface, removing developer particles adhering in non-image areas on said surface by washing said surface with insulating carrier liquid which is relatively devoid of developer particles, and heating said surface to fuse said visible image thereon.

8. Apparatus for developing an electrostatic image on an insulating surface comprising first roller means for applying to said surface a developer mixture of electroscopic developer particles dispersed in an insulating carrier liquid, said first roller means having a peripheral surface adapted to retain thereon a quantity of said developer means, second roller means for applying to said surface insulating carrier liquid relatively devoid of developer particles, means for transporting said insulating surface past said first roller means in close proximity to the peri pheral surface `thereof and past said second roller means substantially in contact with the surface thereof, and a tray adapted to contain a quantity of said developer mixture, both said first and said second roller means being mounted in and near opposite ends of said tray, the end of Said tray in which said rst roller means is mounted having a greater depth than the end of said -tray in which said said second roller means is mounted to enhance settling of said developer particles in the end of said tray wherein said first roller means is mounted.

9. The apparatus of claim 8 whereinthe surface of said rst roller means includes a multiplicity of indentations for enhancing its capability of retaining a quantity of said developer mixture on its surface.

10. Apparatus for developing a latent image on an insulating surface comprising a tray adapted tocontain a quantity of a developer mixture comprising electroscopic developer particles dispersed in an insulating carrier liquid, a developer cylinder rotatably mounted in and near one end of said tray and having on its surface indentations for agitatifng said developer mixture and for enhancing the capability of said developer cylinder for retaining on its surface a quantity of said developer mixture, a second cylinder rotatably mounted in and near the other end of said tray at a substantially lesser depth than said developer cylinder and having a substantially smooth surface, the end of said tray in which said developer cylinder is mounted having a greater depth than the end of said tray in which said second cylinder is mounted to enhance settling -of said developer particles in the end of said tray wherein said developer cylinder is mounted, and means for transporting said insulating surface past said developer cylinder in close proximity to the surface thereof and past said second cylinder substantially in contact with the surface thereof whereby upon rotation of said developer cylinder developer mixture is applied to said insulating surface and upon rotation of said second cylinder said insulating surface is washed with carrier liquid relatively devoid of developer particles, said means cooperating with said second cylinder to squeegee excess developer mixture from said insulating surface.

11. The apparatus of claim 10 wherein said electrostatic image is on an insulating surface of an elongated exible web and said means for transporting said surface includes a rotatable transport cylinder adapted to rotate in concert with said second cylinder and to squeegee said flexible web therebetween to enhance removal of carrier liquid from said insulating surface, said -transport cylinder being positioned relative to said developer cylinder such that said insulating surface on said flexible web is carried by said transport cylinder past said developer cylinder with said insulating surface in close proximity to the surface of said developer cylinder.

12. Apparatus for developing electrostatic images on an electrophotographic recording member comprising an elongated flexible web; said apparatus comprising a fluted developer cylinder for applying to said electrophotographic member a mixture of electroscopic developer particles dispersed in an insulating carrier liquid; a metallic washing cylinder having a relatively smooth surface for applying to said electrophotographic member carrier liquid relatively devoid of developer particles; a transport cylinder positioned above said developer cylinder and said washing cylinder having at least its surface lcomprised of resilient material, said transport cylinder having its surface in contact with the surface of said washing cylinder and spaced about 1/16" from the surface of said developer cylinder, a liquid retaining -tray positioned beneath said cylinders and having said cylinders mounted therein such that said developer cylinder projects into said tray to a substantially greater depth than said washing cylinder, said tray having a slanting oor to provide a greater depth in said tray adjacent said developer cylinder than adjacent said Washing cylinder.

References Cited by the Examiner UNITED STATES PATENTS 1,957,942 5/1934 `Convers 117-102 X 2,229,621 1/1941 Bradner 117-102 X 15 2,362,510 11/1944 Stutsman 117-102 X 2,618,552 11/1952 Wise 96-1 X 2,811,465 10/1957 Greig 117-17.5 2,851,373 9/1958 Tregay et al. l17-17.5 2,877,133 3/1959 Mayer 118-637 X OTHER REFERENCES Dow Corning Silicone Notebook, Fluid Series No. 3,

l() September 1948, pp. 3, 7, 18 and 19.

Bertelsen: IBM Technical Disclosure Bulletin, vol. 1, No. 4, p. 21, December 1958.

Metcalfe et al.: Journal of Oil and Colour Chemists Assn, vol. 39, No. 1l, pp. 845-856, 1956.

WILLIAM D. MARTIN, Primary Examiner.

MILTON STERMAN, JOSEPH B. SPENCER, RICHARD D. NEVIUS, N. G. TORCHIN,

Examiners.

20 M. SOFOCLEOUS, Assistanl Examiner. 

1. IN ELECTROSTATIC PRINTING A METHOD OF DEVELOPING AN ELECTROSTATIC IMAGE ON AN INSULATING SURFACE COMPRISING THE STEPS OF APPLYING TO SURFACE A MIXTURE COMPRISING ELECTROSCOPIC DEVELOPER PARTICLES DISPERSED IN A INSULATING CARRIER LIQUID WHEREBY SAID DEVELOPER PARTICLES ARE ATTRACTED TO SAID ELECTROSTATIC IMAGE AND THEN REMOVING DE- 